It is known to fabricate a thermal IR source on a silicon substrate consisting of a micro-heater formed within a thin membrane layer (made of electrically insulating layers) that is formed by etching part of the substrate. Such devices can be used to provide heat (e.g. 600° C.) with low power consumption (typically from a few mW to hundreds of mW) for use as infra-red sources/emitters.
For Example, Parameswaran et. al. “Micro-machined thermal emitter from a commercial CMOS process,” IEEE EDL 1991 reports a polysilicon heater for IR applications made in CMOS technology, with a front side etch to suspend the heater and hence reduce power consumption.
Similarly, D. Bauer et. Al. “Design and fabrication of a thermal infrared emitter” Sens & Act A 1996, also describes an IR source using a suspended polysilicon heater although the device is not envisaged to be fabricated in a CMOS process. Moreover, wafer bonding is used to encapsulate the heater in vacuum (which adds extra fabrication steps and increases the manufacturing cost).
Patent U.S. Pat. No. 5,285,131 by Muller et al. and patent US2008/0272389 by Rogne et. al both describe similar devices using a polysilicon heater.
San et. al. “A silicon micromachined infrared emitter based on SOI wafer” (Proc of SPIE 2007) describe an IR emitter fabricated from an SOI substrate using polysilicon as the heater and DRIE to form the membrane.
The use of polysilicon in all these designs reduces the stability of the device as polysilicon resistance drifts in time at high temperatures above 400° C.
Yuasa et. al “Single Crystal Silicon Micromachined Pulsed Infrared Light Source” Transducers 1997, describe an infrared emitter using a suspended boron doped single crystal silicon heater. The paper does not envisage the device to be fabricated within a CMOS process.
Watanabe, in patent EP2056337 describes a suspended silicon filament as an IR source. The device is vacuum sealed by bonding a second substrate. This device is not envisaged to be fabricated in a CMOS process, and the construction of the device also does not lend itself to be fabricated in a CMOS process.
Cole et. al. “Monolithic Two-Dimensional Arrays of Micromachined Microstructures for Infrared Applications” (proc of IEEE 1998) describe an IR source on top of CMOS processed device. These IR sources consist of a suspended micro-heater fabricated after considerable post-CMOS processing. These extra processing steps add to the fabrication cost of the device.
Hildenbrand et. al. “Micromachined Mid-Infrared Emitter for Fast Transient Temperature Operation for Optical Gas Sensing Systems”, IEEE Sensor 2008 Conference, reports on a platinum heater on suspended membrane for IR applications. Platinum is however not CMOS compatible and its use in CMOS foundries is prohibited, as it acts as a deep dopant and can contaminate other CMOS process steps.
Similarly Ji et. Al. “A MEMS IR Thermal Source For NDIR Gas Sensors” (IEEE 2006) and Barritault et. al “Mid-IR source based on a free-standing microhotplate for autonomous CO2 sensing in indoor applications” (Sensors & Actuators A 2011) describe a micromachined IR source based on a platinum heater. Weber et. al. “Improved design for fast modulating IR sources” describe suspended as well as closed membrane designs for IR sources, both using a platinum heater and a membrane consisting of Silicon oxide and silicon nitride layers.
Spannhake et. Al. “High-temperature MEMS Heater Platforms: Long-term Performance of Metal and Semiconductor Heater Materials” (Sensors 2006) describes micro-hotplate based on either platinum or antimony doped Tin oxide heaters.
As already mentioned, Platinum is incompatible with CMOS processes and so these devices cannot be fabricated in a CMOS process. This increases the fabrication cost and means that circuitry cannot be fabricated with the device.
Tu et. al, “Micromachined, silicon filament light source for spectrophotometric microsystems” Applied Optics, 2002, presents design of a light source employing single crystal silicon heaters on an SOI membrane. Suspended filaments however, have less mechanical stability than a full membrane.
U.S. Pat. No. 6,297,511 by Syllaios et. al. describes an IR emitter made on a suspended membrane with a resistive heater which can be of various materials such as titanium, tungsten, nickel, single crystal silicon or polysilicon. U.S. Pat. Nos. 5,500,569, 5,644,676, 5,827,438 by Bloomberg et. al. report on IR sources with either polysilicon or metal (such as tungsten, tantalum, titanium-tungsten alloy, molybdenum) heaters. However, these devices are not envisaged to be fabricated using a CMOS process.
WO 02/080620 A1 by Pollien et. al. suggests using metal silicides as the heater material in micro-hotplates. The silicide is mentioned as having a polycrystalline structure from silicides of tantalum, zirconium, tungsten, molybdenum, niobium and hafnium. The possible use of such devices as IR sources is mentioned. However metal silicides are not standard materials used in commercial CMOS processes. Advantages of manufacturing the micro-hotplates by a standard CMOS process are given, however no mention is made of how this can be achieved given that metal silicides is not a material found in CMOS processes. In addition no mention of a CMOS process is made in the claims of the patent.
It is also known to fabricate IR detectors in silicon technology. Kim et. al. “A new uncooled thermal infrared detector using silicon diode” Sens & Act A 89 (2001) 22-27 describes a diode for use as an IR detector. U.S. Pat. No. 6,597,051 describes a thermopile fabricated by micromachining for use as an IR detector. Eminoglu et. al. “Low-cost uncooled infrared detectors in CMOS process” describes an IR detector using diodes on a microbridge membrane fabricated in a CMOS process Sens & Act A 109 (2003) 102-113. A. Graf et. al. “Review of micromachined thermopilers for infrared detection,” Meas. Sci. Technol. 18(2007) R59-R75) describes various thermopile based micro-machined IR detectors reported in literature. It is also known to make NDIR sensors, for example Fordl and Tille “A High-Precision NDIR CO2 gas sensor for automotive applications” IEEE Sensors Journal vol 6 No.6 2006, and patent US2007/0102639 by Cutler et. al describe typical NDIR sensors consisting of a filament bulb as an IR source, and a thermopile based IR detector. The two are placed at the opposite ends of a small chamber where gas can enter through a semi permeable membrane (which blocks dust and IR radiation from outside). Depending on the concentration of the target gas, the amount of IR emission of a particular wavelength is absorbed within the optical path, and using the measurement from the IR detector can be used to determine the gas concentration. Most NDIR sensors also have an optical filter to allow only a small range of wavelengths to reach the IR detector so as to make it specific for the gas that absorbs that wavelength.
Other patents, such as US2008/0239322 by Hodgkinson et. al., U.S. Pat. No. 7,244,939 by Stuttard et. al, US2008/0308733 by Doncaster et. al., and U.S. Pat. No. 7,541,587 by Cutler et al. describe similar devices.
In almost every case, the IR emitter and detector are two different components but packaged together. An exception is U.S. Pat. No. 5,834,777 by Wong, where both the emitter and detector are on the same chip with an optical path made on the chip. However in this case, because the optical path is on the chip, it is a very small distance for the IR emission to travel, and so the sensor has a low sensitivity.